15 photos show a US nuclear missile silo that for decades was ready to strike the Soviet Union at a moment's notice

From the outside, the Titan II missile museum doesn't look like much — just a small building housing the gift shop, a few dopplers outside, and a dust-colored steel mound covering the missile underneath.

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But, after a short introductory video inside the main building, visitors embark on a guided tour in the control room and the hidden silo itself, which reaches 147 feet underground.

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When the silo was operational, personnel on duty descended into the control room through the access portal and into the entrapment area, where they had to confirm their clearance to access the site using a code spoken through a telephone like the one below.

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Four crew members were on duty at all times in the silo. Each crew member served a 24-hour shift, and no crew member could be left alone during the shift because of the classified activity at the site.

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The 24-hour clock in the control room was set to Zulu, or Greenwich Mean Time, and had to be rewound manually every eight days. It's still ticking for visitors.

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The control room, where crew members awaited a phone call from the National Command Authority telling them to launch the Titan II missile, looks exactly as it did when the site was commissioned in 1963.

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The facility, one of 18 in the Tucson area and 54 total in the US, became operational in 1963, and was deactivated in 1982 during then-President Ronald Reagan's effort to upgrade the US's nuclear weapons. The other facilities were in the areas surrounding Little Rock, Arkansas, and McConnell Air Force Base in Kansas.

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The control room is separated from the outer wall of the facility by 11 inches of highly engineered shock absorbers so that in the event of a nuclear blast or some other type of explosion, the crew members in the control room wouldn't even spill their coffee, according to tour guide Jim Sprigg.

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The missile itself was launched from the control room by two crew members simultaneously turning their launch keys at their control stations. Fifty-eight seconds after the keys were turned, the missile would launch, "and no human could stop it," Sprigg said.

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At the Sahuarita facility, the missile's destination was Target 2 — and none of the crew members knew where that was. The information is still classified to this day.

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The Titan II would reach its target destination 30 to 35 minutes after it was launched. Powered by 43,000 pounds of thrust, the missile had a yield of nine megatons — about 250 times the yield of the bombs dropped on Hiroshima and Nagasaki during World War II.

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This drawer held the launch keys for the missile, and the locks on either side of the drawer were considered classified equipment.

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Target 2 was designated as a ground burst, meaning the Titan II at Sahuarita was intended to destroy a facility underground by concentrating its explosive force downward. Its hypergolic propellants — nitrogen tetroxide and unsymmetrical dimethyl hydrazine — ignited only when they combined, creating a fast and reliable detonation.

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Several scenes from Star Trek: First Contact were filmed here at the museum. It's one of only two such museums in the US — the other being the Minuteman Missile Silo in South Dakota.

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The crew on duty inspected the missile silo facilities top to bottom each day — a process that could take three to four hours.

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